[11/17] Readahead support for the variable order page cache

Readahead is now dependent on the page size. For larger page sizes we want less readahead. Add a parameter to max_sane_readahead specifying the page order and update the code in mm/readahead.c to be aware of variant page sizes. Mark the 2M readahead constant as a potential future problem.

[11/17] Readahead support for the variable order page cache

Readahead is now dependent on the page size. For larger page sizes we want less readahead. Add a parameter to max_sane_readahead specifying the page order and update the code in mm/readahead.c to be aware of variant page sizes. Mark the 2M readahead constant as a potential future problem.